Though he put up a good front, Jeff hated being dragged around the galaxy on one archaeological expedition after another. While the scientific and historical aspects of those journeys were not to his liking, Jeff became very "street smart," taking cues father's formative years. (PDN: "True North, "The Forgeman")

While Tyler loved the archaeology, Jeff talked his father into training him in martial arts, and became proficient in several forms. (PDN: "The Forgeman")

After their time on Vulcan, Jeff and Tyler went to live on Deep Space 9, to be closer to their father during his mission in the Demilitarized Zone. On DS9, they spent a lot of time with their mother's foster brother, Miles O'Brien and his family. Much to the O'Brien's dismay, Jeff also spent hours upon hours in Quark's bar, and learned a lot of "underhanded tactics" from both Quark and ConstableOdo.

From the start, Jeff had disciplinary problems at the Academy, but through Tuvok's influence, he worked them out, and became a model cadet. His major at the Academy was security and tactics.

In 2381, Nebula Squadron went on field assignment to the Rigel system. While Tyler spent most of his time investigating the history of the area and places of archaeological interest, Jeff began covertly observing the local underworld.

On Rigel II, Jeff began frequenting a cabaret and became involved with Alareh, a dancer there. Tyler cautioned him against getting in over his head, but Jeff didn't listen, and though he initially resisted, he gave in to the dancer's advances and the spent several nights together. The day before they left Rigel, Jeff returned to visit Alareh, but she rebuffed him, and he was crushed.

In May 2384, mere weeks before graduation, Jeff founds himself troubled by strange dreams (induced by Darrum). Soon after, an unusual chain of events began: his best friend was nearly killed in a shuttle accident from which Jeff walked away unscathed; he was falsely accused of gambling illegally with members of the Orion Syndicate; his girlfriend dumped him for another of his friends; and there was a sudden and complete breakdown of communication with his twin brother. To make matters more complicated, he received a message from Rigel II, informing him of Alareh's death, and the existence of his nearly three year old son, Varen. Ashamed of his indiscretion, Jeff tried to hide the facts but after receiving advice from Commander Tuvok and Ben Bartholomew, he accepted responsibility, confessed fully to his father and brother (via letter), and on the day before graduation, droppped out of the Academy to take custody of his son. (PDN: "Sometimes It Comes In the Clouds")

When the Colonial Alliance was formed in late 2385, Jeff joined the Colonial Navy, believing that to be the way he could best protect his son. (PDN: "Promised Land", "Above the Wrecks of Time", "The Hand of God")

Being an identical twin, Jeff was of course very close to his brother, Tyler. As children, they did everything together, and even shared a strong telepathic link, learning to communicate with each other in that manner by the time they reached age five. Though they had some friends their own age, especially Jeff, both boys were like their father in that they preferred their own company, or that of grownups, over other children. Jeff, the stronger of the two, both physically and telepathically, always felt protective of Tyler, and they leaned on each other heavily after their mother's death. Jeff and Tyler remained close all throughout their formative years and well into their time at Starfleet Academy, until the incident on Rigel II, when Jeff began to withdraw from their mental link, and they slowly drifted apart until Jeff left the Academy.

In one timeline, Jeff and Tyler were on a mid-year break from Starfleet Academy during their freshman year when the Borg launched a full-scale invasion of the Alpha Quadrant in January, 2381. They and their classmate Samok were on Vulcan when it was devastated by Borg cubes. Though they were in the L-langon mountains and distant enough from the major population centers to survive the attack, both all three young men were struck by the psychic death throes of millions of Vulcans. They spent the next several weeks assisting the planetary recovery efforts before resuming their studies at the Academy.

Due to Borg attacks on Rigel, no cadets were sent on training missions there in 2381, thus Jeff did not meet Alareh and the rest of his life was presumably very different from that of his Pendragon counterpart.

In at least one other timeline, Jeff had a son named Tristan Jacob Sinclair, born in 2385. Both Jeff and Tristan's mother died in the line of duty sometime prior to 2402.